Referring to FIG. 1, a first conventional secondary air valve of an engine is disposed in a main body 10 of a cylinder head of the engine. The main body 10 is provided with a combustion chamber 101 beneath and an exhaust duct 102 beneath. The exhaust duct 102 has one end communicating with the combustion chamber 101 for discharging an exhaust gas from the combustion chamber 101. The main body 10 has a top provided with a groove 103 while the secondary air valve is fixedly installed above the groove 103. The secondary air valve includes an inlet pipe 11 having an end formed with a pipe opening 111 in communication with the groove 103. A one-way valve 12 including a valve member 121 and a valve opening 122 is installed between the pipe opening 111 and the groove 103 to separate the pipe opening 111 from the groove 103 and form two totally independent spaces. In addition, the main body 10 is internally provided with an air duct 13 configured for communication between the groove 103 and the exhaust duct 102. The air duct 13 is composed of a transverse channel 104, a vertical channel 105 and a slanted channel 106 that communicate with one another. The transverse channel 104 is formed by boring and processing the main body 10 and inserting a blind plug 14 into a bore thus made. Similarly, the vertical and slanted channels 105 and 106 are formed by boring and processing the main body 10 and inserting blind plugs 15 and 16 into corresponding bores thus made. Therefore, the one-way valve 12 is opened toward the exhaust duct 102 only when a suction force sufficient to open the valve member 121 of the one-way valve 12 toward the exhaust duct 102 is generated in the exhaust duct 102. The one-way valve 12 thus opened also introduces a secondary air into the exhaust duct 102 through the pipe opening 111 of the inlet pipe 11 and prevents the exhaust gas in the combustion chamber 101 from flowing back into the inlet pipe 11. Hence, when the secondary air is introduced into the exhaust duct 102 through the inlet pipe 11 and the air duct 13, oxygen in the secondary air is mixed with the exhaust gas. As a result, pollutants in the exhaust gas that have not been completely burnt undergo a further, secondary combustion by virtue of a high temperature of the exhaust gas and with the assistance of the oxygen, thereby reducing the pollutants in the exhaust gas.
However, the first conventional secondary air valve described above has the following defects:
(1) Since the air duct 13 is provided inside the main body 10 and communicates directly with the one-way valve 12, which is spaced from the exhaust duct 102 only by a very short distance, the one-way valve 12 tends to age and deteriorate prematurely due to the high temperature of the exhaust gas released immediately after combustion. This aging and deteriorating process is aggravated by the fact that the exhaust gas is rich in carbon and confirmed within walls of the main body 10 that prevent easy heat dissipation. Moreover, the carbon in the exhaust gas flowing back tends to deposit between the opened valve member 121 and the valve opening 122 so that the valve member 121 becomes stuck and cannot work properly.
(2) The air duct 13, which is composed of the transverse channel 104, the vertical channel 105 and the slanted channel 106, takes up much space in the main body 10 of the cylinder head of the engine and thereby substantially lower the degree of freedom in designing the internal space and structure of the main body 10. Furthermore, the air duct 13 requires a complicated processing procedure that raises the processing and production costs of the cylinder head.
Referring to FIG. 2, a second conventional secondary air valve of an engine includes a secondary inlet valve 3 installed on a plane 23 of a main body 2 of a cylinder head of the engine. The secondary inlet valve 3 includes a valve seat 31, an upper cover 32 and a one-way valve 33. The valve seat 31 is provided with a partition plate 312 for dividing an internal space of the secondary inlet valve 3 into a backflow chamber 311 and a valve member chamber 313. The partition plate 312 is formed with a communication hole 3121 to allow communication between the backflow chamber 311 and the valve member chamber 313. The one-way valve 33 is received in the valve member chamber 313 and covered by the upper cover 32. The valve seat 31 and the upper cover 32 are secured on the plane 23 of the main body 2 with bolts. The upper cover 32 has a top in communication with an end of a secondary air inlet pipe 22. In addition, the main body 2 is provided internally with an air duct 21, whose two ends are connected respectively with the backflow chamber 311 and an exhaust duct 24 in the main body 2. Ambient air entering the secondary air inlet pipe 22 flows sequentially through the upper cover 32, the one-way valve 33, the communication hole 3121, the backflow chamber 311 and the air duct 21, before being discharged via the exhaust duct 24. When an exhaust gas flows back, carbon contained therein will be blocked by the partition plate 312 and retained inside the backflow chamber 311.
The second conventional secondary air valve described above, though capable of reducing backflow of the exhaust gas and thereby preventing failure of the one-way valve 33 due to carbon deposition, is disadvantaged by structural complexity of the valve seat 31. Moreover, the plane 23 through which the valve seat 31 contacts the main body 2 of the cylinder head demands high flatness and high-precision processing, thereby not only raising the difficulty in processing, but also significantly increasing the production cost of the main body 2 and the valve seat 31. In addition, since the valve seat 31 must be divided into the backflow chamber 311 and the valve member chamber 313, and the valve seat 31 must be secured tightly on the main body 2, the degree of freedom in structural design of the valve seat 31 and the main body 2 of the cylinder head is greatly restricted.
Therefore, engine manufacturers of today have endeavored to design a secondary air valve of an engine wherein a one-way valve of the secondary air valve will not experience carbon deposition and premature aging caused by a backflow of a high-temperature exhaust gas from a cylinder head of the engine. It is desired to improve the reliability and service life of the secondary air valve while maintaining a high degree of freedom in designing the cylinder head.